Presently, an isolated DC-DC power converter applied to high power generally adopts a full-bridge phase-shift circuit structure. The full-bridge phase-shift circuit can achieve zero-voltage switching (ZVS). A phase-shift pulse width modulation (PWM) control method of a fixed switching frequency can also be implemented to the full-bridge phase-shift circuit structure. However, the full-bridge phase-shift circuit structure may lose a soft-switch effect when operating under a light load (or empty load) state, and a circulating loss generated in a heavy load operation is great, which leads to decrease of efficiency.
An existing composite DC-DC power converter having both of a phase-shift circuit and a resonant circuit may achieve the soft-switch effect for full load range, and meanwhile decrease the circulating loss. However, the existing composite DC-DC power converter has to be configured with a main transformer and an auxiliary transformer. The existing composite DC-DC power converter provides power to the load through the auxiliary transformer during a freewheeling period, and during a power transmitting period (a period of providing the maximum power), the existing composite DC-DC power converter provides power to the load through the main transformer, so that the main transformer be sufficient to supply a rated power. Namely, a power level of the main transformer is required to match the rated power, so that a volume of the main transformer is larger. The structure of dual-transformer leads to larger volume of the existing composite DC-DC power converter, and causes a poor overall power density.